Environmental restoration and specifically remediation of radioactive contaminants is a field of primary importance in society today. The problem extends to a wide diversity of contaminants, number of sites, and various forms of contaminants, including ground water, storage solution containing various solvents and acidities, and particulates, etc. Remediation of radioactive waste is of prime importance since it is recognized by the public as one of the leading environmental issues of our time.
While removal of a variety of radioisotopes from waste streams in the environment is essential, removal of technetium-99 (.sup.99 Tc) is considered to be among the most important since it makes up a dominant portion of the radioactive nuclides remaining in radioactive wastes after decay periods of hundreds of years (1-5). There is wide world concern about the environmental contamination of .sup.99 Tc. For example, the total deposition from nuclear tests is estimated to be 140 Tbq or 220 Kg. Other sources are related to the different steps in the nuclear fuel cycle. .sup.99 Tc has been and is being produced in nuclear reactors in amount equaling approximately 1 Kg per ton of uranium. Approximately 10% of the mass of the fission products are made up of .sup.99 Tc since its fission yield is 6.13% (2,3,6).
Although the radiotoxicity of .sup.99 Tc is not high, there is major interest in this radionuclide because of its long half life (2.13.times.10.sup.5 years).
For the above reasons, a major goal is to limit the discharge of .sup.99 Tc to the lowest possible level (2,7,8). Therefore it is essential to develop reliable and practical (cost effective) methods to remove .sup.99 Tc from a variety of vehicles, ranging from high-level liquid waste (HLLW) to low levels of radioactivity in ground water.
Integral to the technologies for the removal of .sup.99 Tc from various media is the chemistry of Tc. TcO.sub.4.sup.- is one of the most stable and unreactive forms of .sup.99 Tc. This chemical form is very mobile and exhibits little interaction with inorganic components in the geosphere (2-5,9). Reduced forms of Tc (for example, Tc(IV)) will be strongly absorbed and exhibit minimal migration. Even though the oxidation of TcO.sub.2 (a major chemical found in spent reactor fuel) to TcO.sub.4.sup.- in aerobic aqueous media is relatively slow (approximately 0.03% in 150 days (1-3,7) over many years a large percentage of conversion to TcO.sub.4.sup.- will occur. Because of the low reactivity of TcO.sub.4.sup.-, the task of developing practical methods of removing this anion with a high degree of selectivity is formidable but also quite critical.
Various prior art methods have been used to separate .sup.99 TcO.sub.4.sup.- from HLLW, ground water, and other media (2,6,10). These prior art methods include absorption on activated carbon (5) extraction into organic solvents containing Tc-acid ligands (3,6,9), extraction from liquid media into supported liquid membranes (SLM) (11,12) and sequestration by ion exchange processes (3,9,10,13).
Making .sup.99 TcO.sub.4.sup.- more interactive by reduction to lower oxidation states is possible. However, the addition of reducing agents to complex chemical media makes this approach impractical. Accordingly, there remains a need for the development of technology that maintains simplicity while providing high selectivity and efficiency of removal of .sup.99 TcO.sub.4.sup.- in order to produce a practical solution to this massive problem.
In developing the necessary technology regarding the above mentioned problems, such technologies must be suitable for the separation of .sup.99 TcO.sub.4.sup.- from large quantities and large volumes of media. For example, removal of .sup.99 TcO.sub.4.sup.- by selective extraction by passing large volumes of solutions containing solid support materials on columns with high affinity and specificity for .sup.99 TcO.sub.4.sup.- or by adding suspensions of non-soluble materials, such as polymers, that will bind .sup.99 TcO.sub.4.sup.- which can be sedimented or filtered from liquid systems would be attractive. The essential criteria that these materials must fulfill is the ability to provide a high degree of specificity for .sup.99 TcO.sub.4.sup.- and a high binding affinity. Recent work by inventors of the present invention demonstrated the ability of triphenylphosphinimine ligands to efficiently extract tracer levels of .sup.99m Tc as pertechnetate into organic solvents (Table 1) (14). The resulting product appeared to be an ion pair based on comparisons of the product formed in similar manner with ReO.sub.4.sup.- (15). The data obtained with .sup.99m TcO.sub.4.sup.- suggested that phosphinimine (PN) ligands hold the potential to be used as a basis to design and develop new .sup.99m Tc radiopharmaceutical applications in Nuclear Medicine (14).
Phosphinimine ligands have been polymerized for use in aerospace, implants, encapsulants, and other uses. Phosphines in any form (including monomers, polymers, or attached to other solid support) have not been successfully utilized at the single molecular level and have not before been applied for extraction of .sup.99 TcO.sub.4.sup.- from liquid or solid wastes for environmental remediation.
The present invention provides a solution to the above discussed remediation problem by adapting the phosphinimine ligand technology to be usable for extracting .sup.99 Tc from various types of liquid media.